1. Field of the Invention
This invention relates to an antenna device and a radio wave controlled timepiece.
The antenna device used here comprises an antenna including a core and a coil wound around the core, and a structure that functions in cooperation with the antenna in order to improve receiving performance of the antenna.
2. Description of the Related Art
A radio wave controlled wristwatch which receives a radio wave including time information and automatically correct the current time is well known. In the radio wave controlled wristwatch, a small sized antenna is installed in a predetermined position of an inner space of a wristwatch case.
A structure for improving the receiving sensitivity of a small sized antenna is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 55-91237. In this document, in order to attract more magnetic flux, a sectional area of each one of both end portions is set to be larger than that of a center portion around which a coil is wound in a core of the antenna. And, the antenna having the above described structure constitutes an antenna device.
When the whole wristwatch case is formed of a metal, the appearance of the wristwatch is improved, and the wristwatch looks like a high quality product. However, the metallic wristwatch case functions as a magnetic shield. Thus, in the case where the antenna is installed in an inner space of the metallic wristwatch case, the receiving sensitivity of the antenna may be significantly lowered.
Further, when a magnetic flux generated by the antenna which received a radio wave passes through the metallic wristwatch case, an eddy current may be generated with the wristwatch case. As a result, a loss occurs in the antenna coil and a Q value is lowered, and finally, the receiving sensitivity of the antenna may be significantly lowered.
A structure for preventing occurrence of the eddy current described above in the wristwatch case is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-311715 (and the corresponding U.S. Pat. No. 7,126,548). In this document, a magnetic member for restraining the generation of the eddy current is arranged between the antenna installed in the inner space of the metallic wristwatch case and a surface of the inner space of the wristwatch case. And, the antenna and the magnetic member described above configure the antenna device.